1. Field of the Invention
The present invention relates to an ophthalmologic apparatus adapted for the measurement of radius of curvature or refractive power of a cornea of an eye, degree of astigmatism, base curve of a contact lens etc.
2. Description of the Prior Art
Conventional keratometers, such as Sutcliffe's and Littmann's, project an index onto the cornea of an eye to be examined from the peripheral direction of an objective lens, observe the size of the image reflected by the cornea with a measuring optical system including an image matching system, and determine the radius of curvature or refractive power of the cornea from the amount of displacement of a prism etc. in said image matching system. Such methods however require considerable time for the measurements and inevitably result in an error if the eye to be examined shows rapid movement.
Also Sutcliffe's keratometer can easily identify the distortion of the reflected image caused by corneal astigmatism, tears etc. from the state of the image reflected by the cornea since an index of a circular slit is projected onto the cornea, but errors in the measurements are unavoidable if the relative position between the index and the eye to be examined is not adequate.
On the other hand, Littmann's keratometer can provide exact measurements with limited errors even when the relative position between the index and the eye to be examined is somewhat different from the optimum situation, but is unable to easily identify the state of corneal astigmatism etc. since the index projected onto the cornea of the eye to be examined is only in the direction of a particular meridian.
In order to overcome the aforementioned drawbacks, the present applicant already proposed, in the U.S. patent application Ser. No. 416,355, a structure in which an annular index is projected from an infinite distance onto the cornea of the eye to be examined. The present invention is to provide further improvements on the above-mentioned structure.